This invention relates to erosion lithography and nozzles; and in particular to manufacture of a bonded cathode and electrode structure for microwave triode tubes.
The grid-controlled power amplifier has long been useful for a variety of microwave applications. The L-64 and L-67 types, developed by J. E. Beggs and his associates as a consequence of work sponsored by the United States Army Electronics Command, have extended the range of performance of such devices. These advances were attained through the use of a closely spaced grid-cathode structure operating in the high-vacuum environment of a titanium-ceramic tube structure.
The construction of grid-cathode units with even closer spacing of grid and cathode and capable of high grid dissipation was continued using a grid and a heater which are rigidly bonded to the cathode by an insulating film. Boron nitride (BN) was identified as the preferred insulating material. Chemical vapor deposition (CVD) of BN was developed, and grid patterns with detail as small as 0.002 inch were formed by erosion through a mask with air driven Al.sub.2 O.sub.3 particles. The d-c characteristics of bonded grid tubes showed a high utilization of emission as useful plate current, ability to withstand large positive grid bias, and the option of a high level of current collection or a wide grid-anode gap. See U.S. Pat. Nos. 3,599,031; 3,638,062; and 3,694,260 by J. E. Beggs.
Several significant technical problems remained, potentially blocking the successful development of still further improvements at higher microwave frequencies of a bonded grid triode. One of these was lack of a process for forming grid openings with dimensions as small as 0.001 inch without either undercutting the supporting insulation or shorting out the insulating layer with metal.
The use of erosion lithography for electrical components is shown in U.S. Pat. No. 3,392,052 by J. Davis. The practice has been to use cylindrical nozzles for sand blasting or erosion lithography with powered abrasives, as shown, for example, in U.S. Pat. No. 3,032,930 by S. B. Williams. However, in attempting to use cylindrical nozzles for finely detailed patterns over large areas, it has been the experience that in spite of favorable erosion rates, the patterns would be completely destroyed in some regions and not cut at all in others.
A method of removing photo resist in a partial pressure of a gas, which may be hydrogen, at about 100 degrees C. is shown in U.S. Pat. No. 3,837,856. Other U.S. patents on removing resist are U.S. Pat. Nos. 3,787,239; 3,582,401; 3,458,401; 3,458,312; and 3,676,219.